Sure there are loads of trains between Bruges and Brussels. Just add the one you'd like to take to the app before boarding. The Eurail Pass gives you that flexibility. :)

Seat reservations are not possible on Belgian trains : hop on and sit wherever you want.

Seat reservations and app are completely separate btw. You log each train to the app before boarding and whenever asked you show both the app (ticket) and PDF (reservation).

So I took the entire original journey off of my pass, but didn’t delete it until I was sure this would work. Then I redid each leg separately and added all three legs to my pass (they combined as one travel day), and it appears to have worked. This is all new to me, we’ve never done a rail pass trip before. I have found it very confusing. But one thing I am figuring it out is that it’s much simpler to choose each leg separately and then add each one to the pass…that makes it much easier to change just one.
 

Part of my frustration has been that the train numbers on the reservations weren’t matching the ticket info on my pass.  I have two reservations for this one journey, one for Bordeaux to Paris, and that one is the same train number, and a second reservation for the train from Paris to Brussels, but that number wasn’t the same….so I added that leg manually. I hope that works…i want the ticket train number on my Eurorail pass to match the train number on my reservation paper. 

Yeah I found it better to add each leg separately as well.

There are often 2 units coupled together bearing different numbers. It can be quite confusing, like a lot of SNCF things. It doesn't matter which train you add as long as times match.

Between Paris-Montparnasse and Paris-Nord you'll have to take RER B (not included). Have a sufficient margin for the connection.

It is best not to activate your pass in advance in case of sudden disruptions. Once a pass day has started you cannot cancel it. More info : https://www.seat61.com/how-to-use-a-eurail-pass.htm#how-does-a-eurail-pass-work

Thank you so much…I wish I had had that link before I even started! Great info there. I especially like that I can add other journeys to the rail planner without adding them to the pass, just in case. I was afraid to try that in case one cancelled out the other. 

Another possibly silly question…for various reasons it works better for me to use my iPad rather than my phone. One big factor is the high cost of roaming charges for connecting to the Internet to update the app and check train schedules…I don’t have those costs if I use my iPad. I know it will be more cumbersome at the gates, etc, but other than that, will there be any problem using the iPad to show tickets vs the phone? 

I don't think there would be issues with an iPad.

The app works with offline data btw, you only need to be connected to the internet every 72h. It's good on one side but then that's why it's better to double-check times with the railway companies as you're not updated in case of disruptions, cancellations.

With my iPad, it will be possible to stay connected to the Internet at all hotels and even on the trains (we chose first class specifically so we can recharge our devices at our seats and connect to the Internet to check the trains and get messages en route.)  I don’t pay for Internet access by iPad, but would by phone. Plus my eyesight isn’t so great, it’s easier working on the iPad. This whole trip has been quite a headache for me to put together…my 82-year old husband has a wish list, and I thought it would be fun for him to do it this way. And he’s very excited. But the rail pass is much more complicated than anticipated. The major stressor has been train numbers not matching reservations, and visions of being stranded somewhere along the way…but we’re deciding to wait and deal with that at the train station in Belgium. Your help has been very much appreciated!!! 

The Eurail app is not the most user-friendly or intuitive, but once you get the hang of it things tend to go smoothly.

Any niggling questions about your trip, don’t hesitate to ask on here. There are a lot of knowledgable people who know the quirks of the railway and Eurail!

My one tip, as above, would be to add each train separately, which gives you more control. And to know which trains you need reservations for (eg Bordeaux-Paris) and which you don’t (eg Brussels-Bruges).

Feel free to post the dates/ times/ train numbers if you’d like someone to check for you.